Interferometric stress birefringence measurement

Problems on polarization aberrations in large aperture dynamic interferometry based on the polarization phase shifting technique

The polarization based phase shifting method is an effective way for dynamic measurements. However, when this technique is applied to the measurements of large optics, the interferometric results are easily limited by the birefringence of large optics. The birefringence changes the polarization states of reference light and test light, and brings constant polarization aberrations into the measurement results independent of the phase shifting procedure. In this article, the detailed theoretical analysis on the mechanism of polarization aberration is presented. Afterwards, we propose a new interferometric method to determine the birefringence effects by measuring the transmitted wavefronts of the large optics, which are considered as birefringent samples. Theoretical analysis shows that the polarization error in the linearly polarized system can be corrected by two independent measurements with orthogonal polarization states. The phase retardance can be obtained from the wavefront difference of the transmitted wavefronts when switching the polarization states of the incident lights. The birefringence distribution obtained is used to calibrate the polarization aberrations in the measurement result of a homemade large aperture measurement platform and the correction result is compared with the result via the wavelength tuning phase shifting method. The elimination of the polarization aberrations can be observed in the final results.

Low-level birefringence measurement by cyclic-path polarization interferometer

A modified cyclic-path interferometer is employed for complete measurement of spatially varying birefringence. An expanded and collimated laser beam intercepted by a birefringent specimen is incident on a polarization-masked cube beam splitter, resulting in two mutually orthogonal polarization components propagating along clockwise and counterclockwise directions in the interferometer. These two wavefronts are made to interfere for four specific orientations of an analyzer. Suitable combinations of the interferograms result in determination of the direction of birefringence and its magnitude. Experimental results are presented.

Selective polarization imager for contrast enhancements in remote scattering media

Conventional intensity imaging through turbid media suffers from rapid loss of image contrast due to light scattering from particles or random variations of refractive index. This paper features the development of an active imaging, snapshot, system design and postprocessing algorithms that differentiate between radiation that scatters or reflects from remote, obscured objects and the radiation from the scattering media itself through a combination of polarization difference imaging, channel blurring, and Fourier spatial filtering. The produced sensor acquires and processes image data in real time, yielding improved image contrasts by factors of 10 or greater for dense water vapor obscurants.

White-light channeled imaging polarimeter using broadband polarization gratings

A white-light snapshot channeled linear imaging (CLI) polarimeter is demonstrated by utilizing polarization gratings (PGs). The CLI polarimeter is capable of measuring the two-dimensional distribution of the linear Stokes polarization parameters by incorporating two identical PGs, in series, along the optical axis. In this configuration, the general optical shearing functionality of a uniaxial crystal-based Savart plate is realized. However, unlike a Savart plate, the diffractive nature of the PGs creates a linear dependence of the shear versus wavelength, thus providing broadband functionality. Consequently, by incorporating the PG-based Savart plate into a Savart plate channeled imaging polarimeter, white-light interference fringes can be generated. This enables polarimetric image data to be acquired at shorter exposure times in daylight conditions, making it more appealing over the quasi-monochromatic channeled imaging polarimeters previously described in the literature. Furthermore, the PG-based device offers significantly more compactness, field of view, optical simplicity, and vibration insensitivity than previously described white-light CLI polarimeters based on Sagnac interferometers. Included in this paper are theoretical descriptions of the linear (S(0), S(1), and S(2)) and complete (S(0), S(1), S(2), and S(3)) channeled Stokes imaging polarimeters. Additionally, descriptions of our calibration procedures and our experimental proof of concept CLI system are provided. These are followed by laboratory and outdoor polarimetric measurements of S(0), S(1), and S(2).

Birefringence in gradient-index rod lenses: a direct measurement method and interferometric polarization effects

Gradient-index (GRIN) media can contain stress birefringence resulting from the variation in material composition. Anisotropy in a GRIN rod lens affects ray propagation and can degrade image quality. A technique, believed to be new, for measuring birefringence in GRIN rod lenses has been developed. The change in optical path difference (OPD) for orthogonal polarizations is measured directly. With this method, effects on OPD from standard imaging aberrations are excluded. Birefringence measurements for various GRIN rod samples are presented. The data are compared with results obtained previously by use of a Twyman-Green measurement method. Also, the polarization effects on tilt fringes observed with the direct measurement method and the Twyman-Green method are presented and modeled theoretically. Tilt fringes for large birefringence test cases are also modeled.

Edge localization of subwavelength structures by use of polarization interferometry and extreme-value criteria

A polarization interferometric method is presented for the quantitative microscopy of topographical structures with subwavelength linewidths. A liquid-crystal phase shifter is inserted into the imaging optics of a reflected-light microscope, and the principles of phase-shifting interferometry are applied to measuring the phase and the contrast of the TE-polarized image (E parallel edge) with the TM-polarized image (E perpendicular edge) as the reference. This common-path interferometric method provides selective edge detection for line structures because the polarization difference is localized at the structure edges. Two different threshold criteria for linewidth determination are discussed: distance of the contrast minima and distance of the points of the steepest phase change. Linewidths as small as 300 nm were measured at a 635-nm wavelength. The dependence on the illumination numerical aperture, as well as on the material, the width, and the depth of the structure, is investigated both experimentally and by rigorous numerical simulations.

Dependence of birefringence and residual stress near laser-induced cracks in fused silica on laser fluence and on laser-pulse number

Measurements of birefringence induced in fused-silica specimens by a crack produced by a 351-nm/500-ps Nd:glass laser as a function of laser fluence F(L) and of number of laser shots N are presented. The varying dimensional parameter is found to be the crack depth a and can be put in the form a(mm) = (0.0096 ? 0.0021)N[(F(L)/F(exit/th)) - 1](2/3) with F(L) >/= F(exit/th)(F(exit/th) is the exit-surface damage threshold). The retardance data are converted into units of stress, thus permitting the estimation of residual stress near the crack. The results of the measured residual stress can be cast in the form varsigma(r)(MPa) approximately (. ? .)[(F(L)/F(exit/th)) - 1](1/2) N(2/3) with F(L)>/= F(exit/th). A theoretical model giving the stress field around a crack is developed for comparison and shows reasonable agreement with the experiment. Good agreement with experimental data of others is also obtained. The effect of residual stresses on fracture strength is pointed out. The results obtained show that the presence of birefringence/residual stress in a fused-silica specimen with a crack on its surface has a strong effect on fracture and should be taken into account in any formulation that involves the failure strength of optical components used in inertial-confinement-fusion experiments.